Piston machines of a wide variety of types make use of piston rings to effect a seal between the piston and a wall of the cylinder in which the piston is reciprocable.
Of particular interest to the present invention are so-called dry-running pistons, i.e. pistons for use in gas machines, e.g. compressors, in which the piston ring effects a seal between a high-pressure side and a low-pressure side, the high-pressure side containing exclusively a gas phase. However, to the extent that the principles of the present invention are applicable to other piston machines, the invention should be understood as including these as well.
In general, when pistons are provided with piston rings the pistons can either be of a one-piece construction or multipart type. The latter construction is used when piston parts are assembled together with the rings to form a structure which is to be reciprocable in the cylinder.
When, however, a one-piece piston is provided, the or each piston ring is received in a respective groove which is machined in the periphery of the piston and can be biased outwardly by intrinsic stress or a separate element.
The piston ring usually has an annular shoulder, engaging an annular step on the piston to capture the ring in the piston.
For example, the piston described in the German patent document (open application--Offenlegungsschrift) DE-OS No. 21 60 826 comprises an outwardly stressed or urged piston ring which has a practically L-shaped cross-section, one leg of the L forming the strip which engages a cylindrical surface of the piston ring having generatrices parallel to the axis. This prevents the entire piston, after wear of the sliding surface, from escaping from the piston groove. When such contact between the lock of the piston ring and the abutment formed by the cylindrical surfaces of the piston contact one another, a wear-free labyrinth seal is provided.
A backing ring in the groove serves to fill part of the space within the groove behind the ring and to hold it against the abutment.
As a result of the clearances which are required between the groove, the piston ring and the backing ring, axial play is always present between these elements.
As a consequence of the axial play, relative movement of the piston and the rings is noticeable in the form of oscillation and impact-like contact of the piston ring with a flank of the groove. Particularly when the piston ring is a nonmetallic member this impact can be detrimental and, as the axial play increases, the danger of canting of the piston ring ensues which limits the sealing effectiveness of the assembly.
It is always expensive and time-consuming to replace such piston rings and to repair any damage to the groove. Frequently the deterioration is so great that the parts are irreplaceable. Finally, the machine may remain inoperative for long periods awaiting replacement of the defective parts.